59RT

I recently overhauled my AC system on my 1991.5 D250 and found my cycling switch was inoperable and had been jumpered by the previous owner causing the evaporator to freeze up. I had no luck finding a new switch and found Jim Lane’s approach to add a new relay to be the way to go.

https://www.dieseltruckresource.com/...switch-112028/

Using Jim’s strategy as inspiration, I went a step further and found a cycling switch from 1970s & 80s Mopars that was adjustable. There is an adjustment for what temperature the switch opens and closes.
-4 Seasons/Murray Thermal Switch 35849



As Jim did, I used the switch to drive a relay that would cycle power to the compressor. I used a micro relay to save space.
-DigiKey Micro Relay A122157
-DigiKey Relay Socket PB1871
-DigiKey 0.25 Connector 12-16 AWG A27929CT
-DigiKey 0.187 Connector 15-20 AWG A123013CT

Instead of running a separate switched circuit to the relay, I used the one that was already powering the old switch and wired it as shown in the pdf diagram that's attached. Might need to be tweaked for years other than 1991.5.

I made a plate to mount the relay and switch to the suction hose in the same place as the stock switch.



I also reused the connector off of the old switch so I could simply plug in the new switch to the factory wiring.




I used thermal paste on the cap tube, mounted the switch and plugged it in.






It works great! I still need to play with the temperature adjustment to dial in the vent temps, but the compressor now cycles and doesn’t freeze up anymore! Hope this helps someone in the future!

patdaly

Nice work and welcome to DTR!